1. Field of the Invention
The present invention relates to a transmission right assigning method for assigning a transmission right to one device of a plurality of devices which are connected to a transmission line and a communication system in which the method is employed.
2. Prior Art
Heretofore, there has been known a network system as a Local Area Network (LAN) management system described in Japanese Patent application of Laid-open No. Hei 5-95361. The LAN management system is constructed by interconnecting a plurality of terminal units through a data transmission line for exchanging data between.
In the above conventional LAN management method, one master unit and a plurality of terminal units are connected by a ring-like data transmission line, and the master unit sends a polling frame to the terminal units one by one through the ring-like data transmission line.
Each terminal unit having no data for transmission sends back a response polling frame to the master unit, while each terminal unit having data for transmission sends back a message data frame to the master unit.
When the master unit receives a response polling frame from a certain terminal unit, the master unit sends a polling frame to the next terminal unit. On the other hand, when the master unit receives a message data frame from a certain terminal unit, the master unit sends a message data frame to the terminal unit and then sends a polling frame to the next terminal unit.
In the above conventional Local Area Network (LAN) management system, however, the master unit sends a polling frame to each terminal unit one by one through the ring-like data transmission line, therefore it takes relatively long time to receive response frame, with the result that it is difficult to transmit emergency data to a destination terminal unit quickly.
As the master unit assigns a transmission right to all terminal unit one after another, but some of them having no data for transmission, there is a certain amount of waist time for data transmission. In addition, as a communication between each terminal unit is carried out through the master unit, there is a disadvantage that data communication speed is relatively low.